KVM: VMX: Don't adjust tsc offset forward

Most Intel hosts have a stable tsc, and playing with the offset only
reduces accuracy.  By limiting tsc offset adjustment only to forward updates,
we effectively disable tsc offset adjustment on these hosts.

Signed-off-by: Avi Kivity <avi@qumranet.com>

1 files changed, 6 insertions, 3 deletions

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 7ef710afceba..fb0389d3a8d1 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -519,7 +519,7 @@ static void vmx_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
 {
         struct vcpu_vmx *vmx = to_vmx(vcpu);
         u64 phys_addr = __pa(vmx->vmcs);
-        u64 tsc_this, delta;
+        u64 tsc_this, delta, new_offset;
 
         if (vcpu->cpu != cpu) {
                 vcpu_clear(vmx);
@@ -559,8 +559,11 @@ static void vmx_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
                  * Make sure the time stamp counter is monotonous.
                  */
                 rdtscll(tsc_this);
-                delta = vcpu->arch.host_tsc - tsc_this;
-                vmcs_write64(TSC_OFFSET, vmcs_read64(TSC_OFFSET) + delta);
+                if (tsc_this < vcpu->arch.host_tsc) {
+                        delta = vcpu->arch.host_tsc - tsc_this;
+                        new_offset = vmcs_read64(TSC_OFFSET) + delta;
+                        vmcs_write64(TSC_OFFSET, new_offset);
+                }
         }
 }